This invention relates to sensors, and more particularly to an object tracking device using waveguide display using electrically switchable gratings.
The tracking of objects is a key requirement in many fields including eye tracking (in augmented reality (AR), virtual reality (VR) and other display applications), robotics, collision avoidance systems and many others. Although the nature of the objects and their dynamics varies greatly there is a general requirement to track robustly, accurately and with minimal processing time lag (latency). Trackers are normally designed to operate in the infrared which offers the benefit of invisibility and can be made eye safe by operating at wavelengths around 1550 nm. Since the tracker will often be used with another device such as a display or some other type of sensor it is highly desirable that the tracker is transparent. The present application is motivated by the need for an improved eye tracker for use in HMDs and most of the embodiments to be disclosed will described in relation to eye tracking. The prerequisite for tracking an object is that it provides a detectable signature from one or more of its surfaces. The signature may be specular reflection, scatter, laser speckle or a combination of these. The object may contain multiple surfaces, for example, in the case of an eye the signature may be provided by surfaces of the cornea, lens and retina. In eye trackers the motion of the eye is detected relative to the sensor. In other tracking applications, such as robot vehicles, the detector may move relative to fixed. In high data content displays, such as those used in AR and VR, eye tracking is essential to reduce latency, the primary cause of motion sickness. Eye tracking enables foveated rendering, a process that limit the amount of image content to be computed and displayed at any time to that lying within the eye's foveal region. Eye tracking is also the key to solving the well-known vergence-accommodation problem that occurs in stereoscopic displays.
Eye tracking is important in Head Mounted Displays (HMDs) because it can extend the ability of the user to designate targets well beyond the head mobility limits. Eye tracking technology based on projecting IR light into the users eye and utilizing the primary Purkinje reflections (from the cornea and lens surfaces) and the pupil-masked retina reflection have been around since the 1980's. The general strategy is to track the relative motion of these images in order to establish a vector characterizing the point of regard. The cornea, which has an aspheric shape of smaller radius than the eye-ball, provides a reflection that tracks fairly well with angular motion until the reflected image falls off the edge of the cornea and onto the sclera. Most solutions rely on projecting IR light into the user's eye and tracking the reflections from the principal surfaces, that at least one surface of the lens, cornea and retina. The first practical challenge is how to introduce the image sensor and illuminator in such a way that both can work efficiently while avoiding obscuring the line of sight Most eye tracker implementations in HMDs have employed flat beam splitters in front of the users' eyes and relatively large optics to image the reflections onto an imaging sensor. Inevitably there are tradeoffs between exit pupil, field of view and ergonomics. The exit pupil is generally limited by either the beamsplitter size or the first lens of the imaging optics. In order to maximize the exit pupil, the imaging optics are positioned close to the beamsplitter, and represent a vision obscuration and a safety hazard. Another known limitation with eye trackers is the field of view, which is generally limited by the illumination scheme in combination with the geometry of the reflected images. The size of the corneal reflected angles would ordinarily require a large angular separation between the illumination and detection optical axes making using corneal reflections over large FOVs very difficult. Ideally, the eye tracker should minimise the angle between the illumination and reflection beams. The temporal resolution of an eye tracker should be at least 60 Hz. However, 90-120 Hz is preferred. Direct imaging by miniature cameras is becoming more attractive as camera get smaller and their resolution increases. However, the latency incurred by the need to recognize and track eye features remains a significant processing bottleneck. From the optical and ergonomic perspective providing a line-of-sight for a camera in a HMD is not trivial. Eye trackers are key components of AR and VR headsets. Desirable an eye tracker should enable the full range of benefits of augmented reality AR and VR displays, namely: a compact and lightweight form factor for encumbrance-free, see-through, mobile and extended use; wide field of view to allow meaningful connections between real world and computer generated images; and the capability of providing robust depth and occlusion cues. The latter are often one of the strongest depth cues. Although recent advances in displays have collectively spanned these requirements no one display technology possesses all of these characteristics.
The inventors have found that diffractive optical elements offer a route to providing compact, transparent, wide field of view eye trackers. One important class of diffractive optical elements is based on Switchable Bragg Gratings (SBGs). SBGs are fabricated by first placing a thin film of a mixture of photopolymerizable monomers and liquid crystal material between parallel glass plates. One or both glass plates support electrodes, typically transparent indium tin oxide films, for applying an electric field across the film. A volume phase grating is then recorded by illuminating the liquid material (often referred to as the syrup) with two mutually coherent laser beams, which interfere to form a slanted fringe grating structure. During the recording process, the monomers polymerize and the mixture undergoes a phase separation, creating regions densely populated by liquid crystal micro-droplets, interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating. The resulting volume phase grating can exhibit very high diffraction efficiency, which may be controlled by the magnitude of the electric field applied across the film. When an electric field is applied to the grating via transparent electrodes, the natural orientation of the LC droplets is changed causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to very low levels. Note that the diffraction efficiency of the device can be adjusted, by means of the applied voltage, over a continuous range. The device exhibits near 100% efficiency with no voltage applied and essentially zero efficiency with a sufficiently high voltage applied. In certain types of HPDLC devices magnetic fields may be used to control the LC orientation. In certain types of HPDLC phase separation of the LC material from the polymer may be accomplished to such a degree that no discernible droplet structure results. SBGs may be used to provide transmission or reflection gratings for free space applications. SBGs may be implemented as waveguide devices in which the HPDLC forms either the waveguide core or an evanescently coupled layer in proximity to the waveguide. The parallel glass plates used to form the HPDLC cell provide a total internal reflection (TIR) light guiding structure. Light is “coupled” out of the SBG when the switchable grating diffracts the light at an angle beyond the TIR condition. Waveguides are currently of interest in a range of display and sensor applications. Although much of the earlier work on HPDLC has been directed at reflection holograms, transmission devices have proved to be much more versatile as optical system building blocks. Typically, the HPDLC used in SBGs comprise liquid crystal (LC), monomers, photoinitiator dyes, and coinitiators. The mixture frequently includes a surfactant. The patent and scientific literature contains many examples of material systems and processes that may be used to fabricate SBGs. Two fundamental patents are: U.S. Pat. No. 5,942,157 by Sutherland, and U.S. Pat. No. 5,751,452 by Tanaka et al. Both filings describe monomer and liquid crystal material combinations suitable for fabricating SBG devices. One of the known attributes of transmission SBGs is that the LC molecules tend to align normal to the grating fringe planes. The effect of the LC molecule alignment is that transmission SBGs efficiently diffract P polarized light (ie light with the polarization vector in the plane of incidence) but have nearly zero diffraction efficiency for S polarized light (ie light with the polarization vector normal to the plane of incidence. Transmission SBGs may not be used at near-grazing incidence as the diffraction efficiency of any grating for P polarization falls to zero when the included angle between the incident and reflected light is small.
There is a requirement for a compact, lightweight, transparent tracker with low latency and a wide field of view for tracking the relative motion of the tracker and one or more objects.
There is a requirement for a compact, lightweight, transparent tracker with low latency and a wide field of view for use in an eye-slaved display.
There is a requirement for a compact, lightweight, transparent tracker with low latency and a wide field of view for use in an eye-slaved display capable of delivering robust depth and occlusion visual cues.
There is a requirement for a compact, lightweight, transparent tracker with low latency and a wide field of view for use in a LIDAR system.
There is a requirement for a compact lightweight transparent display and a wide field of view that integrates a low latency eye tracker and a waveguide display